Thrombosis in the cardiovascular system, particularly of the arteries, veins and heart, causes many ailments, for example, stroke, heart attacks, claudication, deep vein thrombosis, and pulmonary embolism. In particular, stroke clearly compromises quality of life for its victims and for society at large. In economic terms, the total cost of stroke reaches billions of dollars each year. Direct costs include costs associated with hospital and nursing home stays, treatment by physicians and health professionals, drugs, and home health costs and other medical durables. Indirect costs include costs resulting from lost productivity due to morbidity and lost productivity due to mortality. 1995 data for Americans age 40 and older showed the average in-hospital and physician costs were $11,010 for a stroke and $4,940 for trans-ischemic attack (TIA).
Technological advances in the last 60 years to understand and prevent strokes have included angiography, association of carotid disease with cerebral infarcts, carotid endarterectomy surgery, prosthetic heart valves, Doppler ultrasound as a diagnostic modality, use of aspirin to prevent clotting, computed tomography (CT) scanning for differential diagnosis, transcranial Doppler (TCD), and stents for treating arterial lesions. None of the many major advances prior to 1995 were in the realm of direct therapy for stroke at its initiation—it was not until 1995 that techniques involving the administration of thrombolytic agents such as tissue plasminogen activators (t-PA) were shown effective in reducing neurologic damage.
There are two common types of stroke. Embolic occlusion generally accounts for 80% of all strokes, and its treatment is grossly different than that for intracerebral hemorrhage (ICH). The origins of embolic stroke are broadly divided into several categories: thrombus dislodged from a variety of sources including ulcerated carotid or aortic plaque, thrombus of cardiac origin, and thrombus of paradoxical origin from the venous system.
Initial diagnosis of stroke requires radiologic imaging techniques, such as CT or magnetic resonance imaging (MRI), to differentiate between embolic and ICH stroke and to determine the volume of tissue which has had ischemic injury. Generally, a stroke patient is a candidate for thrombolytic therapy if ICH is ruled out, and CT or MRI determines that ischemic changes do not exceed a third of the middle cerebral artery (MCA) territory, blood pressure is normal or controllable, and the diagnosis is made within three hours of onset. Widespread adoption of t-PA therapy under these circumstances has not occurred, however, because hospitals in the United States do not emphasize urgent treatment of stroke patients, and public awareness of the urgent need for medical intervention in stroke is not high. Additionally, there is a shying away from t-PA therapy due to a ten-fold increased risk of ICH, even though this risk is not associated with an overall increase in mortality. All told, only approximately 2% of those stroke patients eligible for t-PA therapy receive it.
Although ultrasound has been available as a diagnostic modality to assess cerebral hemodynamics, and although research in recent years has shown that ultrasound as a therapeutic modality for non-transcranial applications can enhance clot lysis when used in the presence of t-PA, it is not standard practice to use ultrasound to monitor cerebral blood flow during the early stages of stroke or to use ultrasound to treat stroke victims. One reason for the lack of use is due to the high skill level required to acquire and interpret TCD signals using equipment presently on the market and in clinical use. This skill level is not generally available in hospital emergency rooms. There is also a lack of appreciation for TCD capabilities in triage and monitoring. For example, the use of ultrasound monitoring to determine the point in time at which thrombolytic therapy re-establishes perfusion (e.g. blood flow through a vessel) is not fully appreciated.
Another reason for the lack of use of ultrasound is that existing instruments cannot conveniently transmit both diagnostic/monitoring and therapeutic ultrasound. Ultrasound is widespread for diagnosis of these illnesses associated with thrombosis, and a device which performs this diagnostic and the new therapeutic modalities would be of clinical use. That is, standard ultrasound instruments are designed solely for transmission of diagnostic/monitoring ultrasound at a given frequency range and cannot be easily switched to therapeutic applications that may require drastically different frequency ranges, lengths of time of application, beam profiles/coverage, or power levels. Accordingly, not only is there is a need for an ultrasound instrument that can be operated by emergency room personnel and that presents easily-interpreted diagnostic blood flow information, but also there is a need for an ultrasound instrument that can be concurrently used to therapeutically treat stroke patients (as well as individuals suffering other types of thrombosis).